Earth-boring tools are commonly used for forming (e.g., drilling and reaming) bore holes or wells (hereinafter “wellbores”) in earth formations. Earth-boring tools include, for example, rotary drill bits, coring bits, eccentric bits, bicenter bits, reamers, underreamers, and mills.
Different types of earth-boring rotary drill bits are known in the art including, for example, fixed-cutter bits (which are often referred to in the art as “drag” bits), rolling-cutter bits (which are often referred to in the art as “rock” bits), diamond-impregnated bits, and hybrid bits (which may include, for example, both fixed cutters and rolling cutters). The drill bit is rotated and advanced into the subterranean formation. As the drill bit rotates, the cutters or abrasive structures thereof cut, crush, shear, and/or abrade away the formation material to form the wellbore.
The drill bit is coupled, either directly or indirectly, to an end of what is referred to in the art as a “drill string,” which comprises a series of elongated tubular segments connected end-to-end that extends into the wellbore from the surface of the formation. Often various tools and components, including the drill bit, may be coupled together at the distal end of the drill string at the bottom of the wellbore being drilled. This assembly of tools and components is referred to in the art as a “bottom hole assembly” (BHA).
The drill bit may be rotated within the wellbore by rotating the drill string from the surface of the formation, or the drill bit may be rotated by coupling the drill bit to a downhole motor, which is also coupled to the drill string and disposed proximate the bottom of the wellbore. The downhole motor may comprise, for example, a hydraulic Moineau-type motor having a shaft, to which the drill bit is attached, that may be caused to rotate by pumping fluid (e.g., drilling mud or fluid) from the surface of the formation down through the center of the drill string, through the hydraulic motor, out from nozzles in the drill bit, and back up to the surface of the formation through the annular space between the outer surface of the drill string and the exposed surface of the formation within the wellbore.
Fixed-cutter drill bits typically include a plurality of cutting elements that are attached to a face of bit body. The bit body may include a plurality of wings or blades, which define fluid courses between the blades. The cutting elements may be secured to the bit body within pockets formed in outer surfaces of the blades. The cutting elements are attached to the bit body in a fixed manner, such that the cutting elements do not move relative to the bit body during drilling. The bit body may be formed from steel or a particle-matrix composite material (e.g., cobalt-cemented tungsten carbide). In embodiments in which the bit body comprises a particle-matrix composite material, the bit body may be attached to a metal alloy (e.g., steel) shank having a threaded end that may be used to attach the bit body and the shank to a drill string. As the fixed-cutter drill bit is rotated within a wellbore, the cutting elements scrape across the surface of the formation and shear away the underlying formation.
The cutting elements used in such earth-boring tools often include polycrystalline diamond cutters (often referred to as “PCDs”), which are cutting elements that include a polycrystalline diamond (PCD) material. Such polycrystalline diamond cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals under conditions of high temperature and high pressure in the presence of a catalyst (such as, for example, cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high temperature/high pressure (or “HTHP”) processes. The cutting element substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide. In such instances, the cobalt (or other catalyst material) in the cutting element substrate may be drawn into the diamond grains or crystals during sintering and serve as a catalyst material for forming a diamond table from the diamond grains or crystals. In other methods, powdered catalyst material may be mixed with the diamond grains or crystals prior to sintering the grains or crystals together in an HTHP process.
Upon formation of a diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the grains or crystals of diamond in the resulting polycrystalline diamond table. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use due to friction at the contact point between the cutting element and the formation. Polycrystalline diamond cutting elements in which the catalyst material remains in the diamond table are generally thermally stable up to a temperature of about 750° Celsius, although internal stress within the polycrystalline diamond table may begin to develop at temperatures exceeding about 350° Celsius. This internal stress is at least partially due to differences in the rates of thermal expansion between the diamond table and the cutting element substrate to which it is bonded. This differential in thermal expansion rates may result in relatively large compressive and tensile stresses at the interface between the diamond table and the substrate, and may cause the diamond table to delaminate from the substrate. At temperatures of about 750° Celsius and above, stresses within the diamond table may increase significantly due to differences in the coefficients of thermal expansion of the diamond material and the catalyst material within the diamond table itself. For example, cobalt thermally expands significantly faster than diamond, which may cause cracks to form and propagate within the diamond table, eventually leading to deterioration of the diamond table and ineffectiveness of the cutting element.
In order to reduce the problems associated with different rates of thermal expansion in polycrystalline diamond cutting elements, so-called “thermally stable” polycrystalline diamond (TSD) cutting elements have been developed. Such a thermally stable polycrystalline diamond cutting element may be formed by leaching the catalyst material (e.g., cobalt) out from interstitial spaces between the diamond grains in the diamond table using, for example, an acid. All of the catalyst material may be removed from the diamond table, or only a portion may be removed. Thermally stable polycrystalline diamond cutting elements in which substantially all catalyst material has been leached from the diamond table have been reported to be thermally stable up to a temperatures of about 1200° Celsius. It has also been reported, however, that such fully leached diamond tables are relatively more brittle and vulnerable to shear, compressive, and tensile stresses than are non-leached diamond tables. In an effort to provide cutting elements having diamond tables that are more thermally stable relative to non-leached diamond tables, but that are also relatively less brittle and vulnerable to shear, compressive, and tensile stresses relative to fully leached diamond tables, cutting elements have been provided that include a diamond table in which only a portion of the catalyst material has been leached from the diamond table.
As the cutting elements of an earth-boring tool wear during use, what is referred to in the art as a “wear scar” or “wear flat” develops on the cutting element. The area of the wear scar on previously known cutting elements increases with continued wear of the cutting element. As the wear scars of the cutting elements increases, the so-called “weight-on-bit” or “WOB” required to achieve any particular depth-of-cut (DOC) into the formation also increases. Eventually, the drilling system may be unable to provide a WOB sufficient to maintain a DOC needed for efficient drilling. At this point, the cutting elements and earth-boring tool are considered dull and replaced with another earth-boring tool having unworn or less worn sharp cutting elements.